Processing plants use control valves in a wide variety of applications such as, for example, controlling product flow in a food processing plant, maintaining fluid levels in large tank farms, etc. Automated control valves are used to manage the product flow or to maintain the fluid levels by functioning like a variable passage. The amount of fluid flowing through a valve body of the automated control valve can be accurately controlled by precise movement of a valve member (e.g., a valve plug). The control valve (e.g., a sliding valve stem valve) may include a valve stem connected via threads to the valve plug at a threaded opening of the valve plug so that the valve stem is oriented substantially perpendicular to the valve plug. The valve stem may be fixed non-rotatably to the valve plug by welding together the valve stem and valve plug. However, the valve stem and valve plug cannot be welded together if either the valve stem or the valve plug are made of a non-weldable material, or if the filler material is incompatible with the base material of the valve stem or the valve plug. Alternatively, the valve stem may be secured non-rotatably to the valve plug by using a drive pin. However, connecting the valve stem to the valve plug by using a drive pin can cause misalignment between the valve stem and the valve plug and affect the integrity of the valve plug.
FIG. 1 is a partially cut-away schematic illustration of a known sliding valve stem assembly 100. An actuator 110 is coupled to a valve stem 140 that extends into a valve assembly 170. FIG. 2 is an enlarged illustration of a portion of the valve assembly 170 of FIG. 1, including the valve stem 140. As more clearly shown in FIG. 2, an end 148 of the valve stem 140 has threads 143 and an angled opening 146. The valve assembly 170 has an inlet 172 communicating with a valve orifice 174 and an outlet 176. Fluid flow through the valve orifice 174 is controlled by a valve member or valve plug 180. The valve plug 180 includes a central through opening 182 with threads 183, valve plug holes 184, and a counter bore 186 that extends at an angle from a valve surface 185 toward a valve surface 187. The threads 143 at the end 148 of the valve stem 140 are received by the threads 183 of the valve plug 180. As can be readily seen in FIG. 2, a pin 190, which is slightly larger in diameter than the counter bore 186, is located in the counter bore 186 of the valve plug 180 and the angled opening 146 of the valve stem 140.
The valve stem 140 is coupled to the valve plug 180 by threading the threads 143 into the threads 183 at the central through opening 182 to position the valve stem 140 substantially perpendicular to the valve plug 180. After the valve stem 140 has been threaded tightly into the through opening 182, the counter bore 186 is drilled into the valve plug 180 and the opening 146 is drilled into the end 148 of the valve stem 140. The pin 190 is then press-fitted into the counter bore 186 and the opening 146 to secure the valve stem 140 to the valve plug 180 to prevent the valve plug 180 from rotating relative to the valve stem 140 (i.e., non-rotatability). The valve stem 140 has solid contact alignment at the engagement of the upper most threads 143 with the upper most threads 183 in the central through opening 182. However, the engagement of the pin 190 with the end 148 of the valve stem 140 is near the valve surface 187. The force of the press fitting of the pin 190 into the valve stem 140 can result in the valve stem 140 being positioned at a non-perpendicular angle (e.g., misaligned) relative to the valve plug 180. The non-perpendicular misalignment of the valve stem 140 relative to the valve plug 180 can affect the integrity of the coupling of the valve plug 180 to the valve stem 140.